Those percentages are weighted by their previous portion of the total value. A down 10% B down 90% If B is 50% of the market, the market is down by like... 5

Those percentages are weighted by their previous portion of the total value. A down 10% B down 90% If B is 50% of the market, the market is down by like... 50% You can construct practically any total within the extrema if you use the right weights.